Process for preparing methacrylic or acrylic esters

ABSTRACT

A process for producing a methacrylic acid ester or an acrylic acid ester comprising: reacting methacrolein or acrolein with an alcohol or molecular oxygen in the presence of a catalyst comprising Pd; removing water with a separation membrane which can selectively permeate water from a mixed liquid of the alcohol and water.

This application is the national phase under 35 U.S.C. §371 of prior PCTInternational Application No. PCT/JP97/03173 which has an Internationalfiling date of Sep. 9, 1997 which designated the United States ofAmerica.

TECHNICAL FIELD

This invention relates to a process for producing a methacrylic acidester or an acrylic acid ester from methacrolein or acrolein, and analcohol and molecular oxygen in the presence of a catalyst comprisingPd.

BACKGROUND ART

A new route is now being spotlighted for producing, in one step, amethacrylic acid ester or an acrylic acid ester by reacting methacroleinor acrolein with an alcohol and molecular oxygen. This reaction iseffected by reacting methacrolein or acrolein with molecular oxygen inan alcohol, in the presence of a catalyst comprising Pd.

This reaction produces water in a similar manner as the conventionalesterification reaction using a carboxylic acid and an alcohol. Some ofthe water produced competes with the alcohol to react with an aldehyde,to produce a carboxylic acid as a by-product, and consequently, theselectivity of a carboxylic acid ester is lowered. Also, products suchas water, carboxylic acids and the like are considered to be easilyadsorbed on the active site of the catalyst, thereby increasinglyreducing the reaction rate as the concentrations of water and carboxylicacid increases. Therefore, attempting to increase the productivity byincreasing the aldehyde concentration without changing the catalystamount, results in decreasing the reaction rate.

An approach to solving the above-mentioned problems associated with highproductivity in the presence of high concentrations of an aldehyde hasbeen to replace the reactor with a multi-stage reactor. More-over, amethod in which the reaction is effected while water is removed from thereaction system has been proposed in JP-B-4-78,626. In this reference,it is disclosed that the reaction is effected while molecular sieves,which are general water-adsorbents, and the like are added to thereaction system as a means for removing water. According to this method,conversion is not reduced even at a high aldehyde concentration, and ahigh selectivity of methyl methacrylate or methyl acrylate is shown.

However, the present inventors have found that in order to continuouslycarry out the reaction by this method, recycling of the molecular sievesis indispensable, that is, the used molecular sieves are taken out ofthe reaction system, regenerated, and introduced again into the reactionsystem. The operation of recycling the absorbent on a commercial scaleresults in a reduction of the running operability, and when the adsorbedwater is removed in the course of regenerating the absorbent, some ofthe methacrylic acid ester or acrylic acid ester produced by thereaction is removed together with the water, resulting in a reduction inyield. Accordingly, this method in which an absorbent, such as amolecular sieve or the like, is used is effective when the method iscarried out on a small scale for a short period of time. However, whenthe method is carried out on a commercial scale, not only is there theneed for absorbent-regenerating process equipment, but also there isproduct lost during the regenerating process. Furthermore, theabove-mentioned method is ideally suited for a batch operation, andhence, is problematic when a continuous reaction is carried out for along period of time.

On the other hand, the present inventors have examined a method usingreaction-distillation in which azeotropic distillation with water iseffected; however, the complete separation of the starting aldehyde andalcohol from the objective carboxylic acid ester has been difficult, andit has been difficult to selectively separate only water bydistillation. That is to say, by the conventional technique, it has beendifficult to continue the reaction while continuously removing waterfrom the reaction system on a commercial scale.

DISCLOSURE OF THE INVENTION

In consideration of the state of the art, the present inventors haveresearched a process for producing a methacrylic acid ester or anacrylic acid ester which process is high in selectivity for theobjective product and can be carried out on a commercial scale. As aresult, they have found that water can be separated from the reactionsystem by using a functional separation membrane which selectivelypermeates water from a mixed liquid of an alcohol with water. Therebycompleting the invention by which a methacrylic acid ester or an acrylicacid ester can be stably and continuously produced while water iscontinuously removed.

That is to say, an embodiment of this invention is a process forproducing a methacrylic acid ester or an acrylic acid ester by reactingmethacrolein or acrolein with an alcohol and molecular oxygen in thepresence of a catalyst comprising Pd, wherein the reaction is effectedwhile water is removed through a separation membrane which canselectively permeate water from a mixed liquid of an alcohol and water.

This invention aims at economically providing a carboxylic acid esterwith high selectivity and productivity by such a production process.

The separation of a mixture of at least two liquids, which have closeboiling points, by distillation is difficult, and the separation of anazeotropic mixture of liquids having the same boiling point bydistillation is impossible, and hence, research on a functional membranewhich can be used for the separation of them has heretofore been made.However, in the reaction system for producing a methacrylic acid esteror an acrylic acid ester according to this invention, it is necessary toseparate water from a mixed liquid comprising many kinds of organicmaterials such as an aldehyde, for example, methacrolein, acrolein orthe like; an alcohol; a methacrylic acid ester or an acrylic acid ester;and a carboxylic acid ester, and in view of the complexity of membraneseparation of an organic liquid mixture, it is difficult to predict thetype of membrane useful for separating water. Therefore, no membrane hasheretofore been used for water separation in such a reaction system. Thepresent invention includes the separation of water from a reactionsystem of a methacrylic acid ester or an acrylic acid ester by use of afunctional separation membrane which selectively permeates water from amixed liquid of water with an alcohol as mentioned above.

According to the process of the present invention, unlike theconventional process using an adsorption method, it is possible tocontinuously remove water at a rate such that the water concentrationcan be maintained low and constant. Accordingly, it has become possibleto obtain a methacrylic acid ester or an acrylic acid ester with a highselectivity and simultaneously maintain over a long period of time, ahigher reaction rate than has been shown by conventional methods.Moreover, since the reaction conditions are maintained constant, theamount of alkali necessary to neutralize the carboxylic acid produced(owing to the water produced as a by-product) can be sharply reduced. Inaddition, though this alkali is neutralized again with sulfuric acid inthe purification process and thereafter removed as a waste, even in thiscase, the amount of the sulfuric acid used there and the amounts of theneutralization product, sodium sulfate, can also be reduced.Furthermore, since the amounts of the carboxylic acid or the alkali andthe like used in the neutralization are reduced, the load applied to acatalyst is decreased and the catalyst life is prolonged. Therefore, thepositive effects obtained by water-removal in the present invention isvery great.

BRIEF DESCRIPTION OF DRAWING

FIG. 1 is a conceptual view of an example of the reaction apparatus usedin this invention.

BEST MODE FOR CARRYING OUT THE INVENTION

In this invention, what can be used as a water-separation membrane is aseparation membrane which can selectively permeate water from a mixedliquid of an alcohol with water. As separation membranes havingexcellent characteristics for separating water from an alcohol, thereare 1) a membrane which separates the alcohol component by selectivepermeation and 2) a membrane which separates water by selectivepermeation. Preferably the membrane which selectively separates water isused.

A water separation membrane is selected based on parameters includingshape of the reactor, and the like. Organic membranes include, but arenot limited to a polyhydroxymethylene membrane, an acrylicacid-acrylonitrile copolymer membrane, an ionized chitosan membrane, acomposite membrane in which a PVA active layer crosslinked with maleicacid is provided and a polymer alloy membrane as mentioned respectivelyin JP-A-59-109,204 and JP-A-60-129,104, and the like; inorganicmembranes such as an A type zeolite membrane as stated in ChemicalEngineering Symposiums, Vol. 41, pages 102-105 (1994) and the like. Alsoincluded are organic and inorganic membranes which have been known aswater-ethanol separation membranes.

From an industrial perspective, the membrane ideally has excellentdurability and excellent chemical resistance to the reaction mixturewhich comprises both acidic materials and alkaline materials, which arefed for neutralizing the said acidic materials, and the like; also itkeeps a sufficient mechanical strength to resist mixing for thethree-phase-mixing of a gas, a liquid and a solid; and the like.Inorganic membranes are generally functional at high workingtemperatures and have excellent chemical resistance.

In the reaction system, swelling of the water-separation membrane withmethacrolein and acrolein may take place. Therefore it is preferable touse water-separation membranes that are high in resistance to swellingwith methacrolein and acrolein and whose membrane performance(separation factor, permeation flux and the like) is not changed evenafter a long time, for example, 100 hours, has elapsed. It is preferablethat the water-separation coefficient (α) in the reaction system, inwhich methacrolein or acrolein is present, is at least 1,000 and thepermeation flux is large. Inorganic membranes having a permeation flux(Q) of at least 0.01, specifically an A type zeolite membrane and thelike are preferably used.

Incidentally, the separation membrane of the present invention which isused in the reaction system for producing various methacrylic acidesters or acrylic acid esters from methacrolein or acrolein and variousalcohols, and the above-mentioned water-separation coefficient (α) andpermeation flux (Q) are determined under the following conditions:

when using a separation membrane having a membrane area of 0.01 m², asolution consisting of 15% by weight of methacrolein, 15% by weight ofmethyl methacrylate, 65% by weight of methanol and 5% by weight of wateris initially vacuum pumped from the permeation side at a temperature of80° C. and an absolute pressure of 3 kg/cm² for 100 hours and issubjected to pervaporation separation, the following equation isobtained:

    α.sub.AB =(Y.sub.A /Y.sub.B)*(X.sub.B /X.sub.A)

wherein A represents water, B represents the other components, X_(A) andX_(B) represent the weight fractions of water A and the other componentsB on the liquid-feeding side, respectively, and Y_(A) and Y_(B)represent the weight fractions of water A and the other components B onthe permeation side as Y_(A) and Y_(B) respectively.

Also, the permeation flux is the permeation weight per unit membranearea per hour and is indicated by the following equation:

    Q=(weight of the overall components on the permeation side)/0.01*100(Kg/m.sup.2.h).

The above-mentioned A type zeolite membrane can be obtained, (asmentioned in Chemical Engineering Symposiums, Vol. 41, pages 102-105(1994)) by immersing a porous aluminum support having a pore diameter ofabout 1 μm as a substrate in a mixed solution comprising sodiumsilicate, sodium hydroxide, sodium aluminate and aluminum hydroxidehaving such a composition that H₂ O/Na₂ O=60, Na₂ O/SiO₂ =1 and SiO₂/Al₂ O₃ =2, and thereafter subjecting them to a hydrothermal reaction ata temperature of 80-100° C. for 3-12 hours. Incidentally, by repeatingthe immersion and the hydrothermal reaction, it is possible to adjustand control the characteristics of the membrane.

The shape, size and the like of the separation membrane are varieddepending upon the size of the reactor and are set depending upon theamount of water to be produced by the reaction of an aldehyde with analcohol or the like, the amount of water to be removed and the membraneperformance.

The separation membrane can be formed into various shapes by selectingthe shape of the substrate and any shape can be selected according tothe structure of the reactor. For example, flat membrane, modularizedflat membrane, cylindrical membrane, modularized cylindrical membraneand the like are generally used.

The water-removing functional portion of the separation membrane may beinstalled into the line where the reaction mixture is recycled, therebyreturning to the reactor, the reaction mixture from which water has beenseparated and removed. Alternatively, it may be installed directly inthe interior of the reactor, or may be set in both the reactionmixture-recycling line and the interior of the reactor. It is preferableto select an appropriate method depending upon the conditions and carryout the same.

As the mode in which the water-removing functional portion is installeddirectly in the interior of the reactor, for example, FIG. 1 ismentioned. In this mode, the starting materials and air are fed througha starting materials-feeding line 4 and an air-feeding line 5,respectively, to the reactor which is maintained at a constanttemperature with a heat transfer medium fed from a heat transfermedium-feeding line 8, and the reaction mixture is taken out from areaction mixture-withdrawing port 6. In the lower part of the reactor, awater-separation membrane 1 is installed so as to contact the reactionmixture side, so that by creating a negative pressure on the oppositeside of the membrane by a vacuum pump 10 (a pervaporation (PV) method),it is possible to carry out the reaction continuously while removingwater. In FIG. 1, 2 refers to a catalyst-separating filter, 3 to coolingcondenser, 7 to a vent line, 9 to a heat transfer medium-withdrawingline and 11 to a stirrer.

When removing water, a high temperature is advantageous from theviewpoint of water-permeating rate, but a low temperature isadvantageous from the viewpoint of water-separating performance. Thetemperature can be selected in the range of from room temperature to200° C. When the water-removing functional portion is incorporated intothe reaction system as a part of the reactor, the reaction temperatureis selected from the range of 50-160° C., preferably from the range of70-120° C.

When the operating pressure for separating water is set to create apressure difference between both sides of the membrane, water-separatingoperation is possible. The pressure is usually set in the range of0.5-20 Kg/cm². When the operating pressure is, for example, 5 Kg/cm²,water can be separated even if the pressure on the opposite side of themembrane is the normal pressure, but in this case, the flow rate issmall. Therefore, it is preferable to use a vacuum, since water can beremoved more effectively.

In the present invention, it is ideal to remove the water until theamount of the remaining water approaches zero, with respect toproduction rate (reaction rate) and selectivity; however, for this to bepossible, a vast membrane area becomes necessary, which is noteconomical. The effect of water removal varies depending upon the kindsof aldehyde and alcohol to be reacted and the reaction conditions, sothat the ideal degree of water removal is not constant for system tosystem. By removing 1/2 of the concentration of water produced, an aboutdouble productivity can be obtained. Therefore, the preferable amount ofwater remaining in the reaction mixture is 1/2 of the amount of waterproduced, and the more preferable amount is 1/3-1/10. Furthermore, theeffect is further increased by reducing the amount to 1/100 or less;however, as mentioned above, in view of the production cost of membraneand equipment costs such as installation space or the like, thepreferable range is from 1/3 to 1/10.

The catalyst used in the reaction is a palladium-containing supportedcatalyst. Preferably, the catalyst contains palladium and lead, and morepreferably, a specific palladium-lead intermetallic compound is used. Acatalyst which satisfies the conditions shown in Examples appearinghereinafter is much more preferable. As different elements frompalladium and lead, there may be contained Hg, Tl, Bi, Te, Ni, Cr, Co,Cd, In, Ta, Cu, Zn, Zr, Hf, W, Mn, Ag, Re, Sb, Sn, Rh, Ru, Ir, Pt, Au,Ti, Al, B, Si and the like.

The catalyst carrier can be broadly selected from silica, alumina,silica-alumina, zeolite, magnesia, magnesium hydroxide, titania, calciumcarbonate, activated carbon and the like.

The amount of palladium supported on a carrier is not particularlylimited; however, it is usually 0.1-20% by weight, preferably 1-10% byweight, based on the weight of the carrier. The amount of lead supportedis neither particularly limited and is usually 0.1-20% by weight,preferably 1-10% by weight. The supported composition ratio (atomicratio) of palladium/lead is rather important than the amount of each ofpalladium and lead supported. The supported composition ratio (atomicratio) of palladium/lead is 3/3-3/0.9, preferably 3/2 to 3/0.9, and morepreferably 3/1.3 to 3/0.9. The elements other than palladium and leadare in the range of 0-5% by weight, preferably not more than 1% byweight.

The amount of the catalyst varies greatly and depends upon the kinds ofthe reactants, the composition of catalyst, the method of preparingcatalyst, the reaction conditions, the reaction type and the like and isnot particularly limited; however, when the catalyst is reacted in theslurry state, it is preferable to use the same in a proportion of0.04-0.5 kg per one liter of the reaction mixture.

An explanation is made below of the present process for continuouslyproducing a methacrylic acid ester or an acrylic acid ester.

Acrolein and methacrolein which are the starting aldehydes used can beused alone or in an admixture.

The starting alcohol is not particularly limited, and various alcoholscan be used as the reactant. Aliphatic alcohols, aromatic alcohols andthe like can be used. Specifically, in the case of the production of amethacrylic acid ester, methanol is used and in the case of an acrylicacid ester, methanol, ethanol, n-butanol, 2-ethylhexanol and the likeare used, whereby the corresponding esters such as methyl methacrylate,methyl acrylate, ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylateand the like can be obtained, respectively.

The ratio of the amounts of the aldehyde and alcohol used is notparticularly limited, and can be selected in such a broad range that,for example, the aldehyde/alcohol mole ratio is 10/1-1/1,000; however,in general, it is selected in the range of 1/1-1/50. More-over, in thisinvention, even when the aldehyde concentration is high and in the rangeof 2/1-1/4, it is possible to realize a sufficiently high selectivity ofmethacrylic acid ester or acrylic acid ester.

The production of a methacrylic acid ester or an acrylic ester can becarried out by any method which has heretofore been known and in which agas phase reaction, a liquid phase reaction, an irrigation reaction orthe like is used. For example, when the reaction is effected in theliquid phase, the production can be carried out in any type of reactorsuch as a bubble column type reactor, a draft tube type reactor, astirring bath type reactor or the like in which a water-selectiveseparation membrane is installed.

Oxygen used in the production of a methacrylic acid ester or an acrylicacid ester can be used in the form of molecular oxygen, namely oxygengas per se, or in the form of a mixed gas in which an oxygen gas isdiluted with a diluent inert to the reaction, for example, nitrogen,carbonic acid gas or the like. As the source of oxygen, air can also beused. The oxygen partial pressure on the outlet side of the reactor ispreferably 0.4 kg/cm² or less though it varies depending upon the kindsof reactants, the reaction conditions, the type of reactor and the like.The oxygen partial pressure can be adjusted to not more than 0.2 kg/cm²,however when oxygen pressure is too low, the conversion of the startingaldehyde is lowered and troublesome by-products are produced, so thatthe oxygen partial pressure should be selected from such a range so asto minimize these adverse effects.

The reaction pressure can be selected from any broad pressure range offrom under reduced pressure to under pressure. Usually, however, it isselected from the range of 0.5-20 kg/cm². It is better to set the totalpressure so that the oxygen concentration in the gas discharged from thereactor does not exceed the explosion limit (8%).

For the production of a methacrylic acid ester or an acrylic acid ester,it is preferable to keep the pH in the reaction system at 6-9 by addingto the reaction system an alkali metal compound or an alkaline earthmetal compound (for example, oxide, hydroxide, carbonate, carboxylicacid salt or the like). The compounds of these alkali metals or alkalineearth metals can be used alone or in combination.

The reaction time is not particularly limited, and varies depending uponthe set conditions, so that it cannot be uniquely determined. It isusually, however, 0.5-20 hours.

The present invention is illustrated with the following Examples andComparative Examples and is not particularly limited thereto.Incidentally, the pressure used in Examples and the like is shown byabsolute pressure and indicated in kg/cm².

The palladium-containing catalyst used in the reaction is preferably apalladium-lead intermetallic compound satisfying the specific conditionsand is a Pd₃ Pb₁ intermetallic compound, the main peak of which iswithin the range of 2θ=38.55-38.70 degree as measured by the X-raydiffraction mentioned below, is preferred from the viewpoint of catalystperformance. Accordingly, a preferable catalyst was prepared accordingto the preparation method shown in the following Reference ProductionExample. Also, the measurement of the Pd₃ Pb₁ intermetallic compound bythe X-ray diffraction was required to be conducted with good precisionand hence carried out according to the measurement procedure mentionedbelow. The catalyst was subjected to measurement after it was vacuumexhausted at 160° C. and then treated for 3 hours to remove the lowmolecular weight adsorbed/occluded components.

Measurement of X-ray Diffraction Angle on the (111) Face ofPalladium-Lead Intermetallic Compound

Using an X-ray diffraction apparatus Model RAD-RA manufactured by RigakuDenki K. K., the diffraction angle 2θ on the (111) face of apalladium/lead intermetallic compound which was a supported catalyst wasmeasured according to the conventional measurement procedure for powderX-ray diffraction using CuKαl ray (1.540-5981). The measurement must beeffected with particularly high precision. The (111) face and (200) faceof the LaB₆ compound defined as the standard reference material 660 in,for example, National Institute of Standards & Technology were measuredand standardized so that the respective peak values became 2θ=37.441degree and 43.506 degree, whereby results with high measurementprecision and good reproducibility are obtained.

REFERENCE PRODUCTION EXAMPLE 1

Aluminum nitrate and magnesium nitrate were add to and dissolved inSNOWTEX N-30 (a trade name of Nissan Chemical Industries, Ltd., SiO₂content: 30% by weight) as an aqueous silica sol solution in suchrespective proportions that Al/(Si+Al)=10 mole % and Mg/(Si±Mg)=10 mole%, and thereafter, they were spray dried in a spray dryer set at atemperature of 130° C. to obtain a spherical carrier having an averageparticle size of 60 μm. It was calcined at 300° C. and then at 600° C.Thereafter, this was used as a carrier and poured into an aqueoussolution having dissolved therein 15% by weight of palladium chlorideand 10% by weight of sodium chloride so that the palladium contentbecame 5 parts by weight per 100 parts by weight of the carrier, theresulting mixture was maintained at 60° C. for 2 hours to completelyadsorb and support Pd on the carrier, and the supernatant was thendumped. Subsequently, water and sodium acetate were added so that a 6%by weight aqueous sodium acetate solution was prepared, and lead acetatewas added in a proportion of 4.2 parts by weight per 100 parts by weightof the carrier, after which hydrazine was dropwise added thereto in aproportion of 3 moles per mole of palladium with stirring at 90° C. Themixture was maintained at the same temperature for a further one hour toobtain a reduction catalyst (written as Pd5.OPb4.2/Mg, Al--SiO₂). ThePd/Pb supported composition ratio of the supported catalyst was 3/1.29by atomic ratio and the X-ray diffraction angle (2θ) on the (111) faceof the palladium/lead intermetallic compound was 38.620 degree.

Example 1

50 g of the catalyst of Reference Production Example 1 was placed in astirring bath type reactor having a liquid phase portion of 400 mlequipped with a catalyst-separator in which a separation membrane (aflat membrane of A type zeolite obtained according to the methoddescribed in Chemical Engineering Symposiums, Vol. 41, pages 102-105(1994) (the effective membrane surface area: 80 Cm², α=8,000, Q=0.25)and a sintered filter made of stainless steel having a pore diameter of2 μm were installed; a vacuum was applied to the opposite side of theseparation membrane; and reaction was conducted while water wasseparated by the PV method. To the reactor were continuously fed a 33.3%by weight methacrolein/methanol solution in which lead acetate wasdissolved so that the lead concentration in the fed starting materialliquid became 10 ppm, at a rate of 0.336 liter/hr and a NaOH/methanolsolution at a rate of 0.037 liter/hr (corresponding to an aldehydeconcentration of about 30% by weight), and the methyl methacrylate(MMA)producing reaction was conducted while the amount of air wascontrolled so that the reaction temperature was 80° C., the reactionpressure was 5 kg/cm² and the outlet oxygen concentration was 4.0%(corresponding to an oxygen partial pressure of 0.20 kg/cm²). Theconcentration of NaOH fed to the reactor was controlled so that the pHof the reaction mixture became 7.1. When 10 hours had elapsed, thereaction product was analyzed to find that the methacrolein conversionwas 51.3% and the selectivity of methyl methacrylate was 93.8%. Thewater concentration of the recovered reaction mixture was 1.65% byweight and the MMA-producing rate per catalyst was 11.64 moles/h.Kgcatalyst. When the reaction was continued for a further 100 hours, themethacrolein conversion was 51.0% and the selectivity of methylmethacrylate was 94.0% and the producing rate was 11.60 moles/h/Kgcatalyst, and substantially no change of reaction performance was seen.

Comparative Example 1

In quite the same apparatus as in Example 1, except that the separationmembrane was not installed, the reaction was conducted under the sameconditions as in Example 1. When 10 hours had elapsed, the reactionproduct was analyzed to find that the methacrolein conversion was 29.8%and the selectivity of methyl methacrylate was 90.3%. The waterconcentration in the recovered reaction mixture was 3.25% by weight, andthe methyl methacrylate-producing rate per catalyst was 6.5 moles/-h.Kgcatalyst.

Example 2

Using an anionic polysaccharide membrane (effective membrane area: 14cm²) prepared by the method described in Example 1 of JP-A-60-129,104 asthe separation membrane, 6 g of the catalyst of Reference ProductionExample 1 was placed in a stirring bath type reactor having a liquidphase portion of 50 ml equipped with a catalyst-separator in which asintered filter made of stainless steel of 2 μm was installed, a vacuumwas applied to the opposite side of the separation membrane and thereaction was carried out while water was separated by the PV method. Tothe reactor were continuously fed a 33.3% by weightmethacrolein/methanol solution in which lead acetate was dissolved sothat the lead concentration in the fed starting material liquid became10 ppm, at a rate of 45 ml/hr and a NaOH/methanol solution at a rate of5 ml/hr (corresponding to an aldehyde concentration of about 30%), andthe MMA-producing reaction was conducted while the amount of air wascontrolled so that the reaction temperature was 80° C., the reactionpressure was 5 kg/cm² and the outlet oxygen concentration was 4.0%(corresponding to an oxygen partial pressure of 0.20 kg/cm²). The NaOHconcentration to be fed to the reaction was controlled so that the pH ofthe reaction mixture became 7.1. When 10 hours had elapsed, the reactionproduct was analyzed to find that the methacrolein conversion was 43.2%and the selectivity of methyl methacrylate was 91.0%. The waterconcentration in the recovered reaction mixture was 2.38% by weight andthe methyl methacrylate-producing rate per catalyst was 9.5 moles/h.Kgcatalyst. The reaction was continued for a further 20 hours. Themethacrolein conversion was 42.6%, the selectivity of methylmethacrylate was 91.2% and the producing rate was 9.4 moles/h/Kgcatalyst, and a constant reaction performance was maintained.

Example 3

The reaction was conducted in the same manner as in Example 1, exceptthat acrolein was substituted for the methacrolein. When 10 hours hadelapsed, the reaction product was analyzed to find that the acroleinconversion was 63.6% and the selectivity of methyl methacrylate was92.2%. The water concentration in the recovered reaction mixture was2.9% by weight and the methyl acrylate-producing rate per catalyst was17.4 moles/h.Kg catalyst. When the reaction was continued for a further100 hours, it was found that the acrolein conversion was 63.1%, theselectivity of methyl acrylate was 93.5%, the producing rate was 17.6moles/h/Kg catalyst, and a constant reaction performance was maintained.

Example 4

The reaction was conducted in the same manner as in Example 3, exceptthat n-butanol was substituted for the methanol and a method in which 10ppm of lead and NaOH were dissolved in and fed to a part of thewithdrawn reaction mixture to control the pH to 7 was adopted. When 10hours had elapsed, the reaction product was analyzed to find that theacrolein conversion was 48.5% and the selectivity of n-butyl acrylatewas 92.8%. The water concentration in the recovered reaction mixture was1.97% by weight and the butyl acrylate-producing rate per catalyst was13.4 moles/h.Kg catalyst. When the reaction was continued for a further100 hours, it was found that the acrolein conversion was 47.3%, theselectivity of butyl acrylate was 93.5%, the producing rate was 13.2moles/h/Kg catalyst, and a constant reaction performance was maintained.

Example 5

The reaction was conducted in the same manner as in Example 4, exceptthat 2-ethylhexyl alcohol was substituted for the methanol. When 10hours had elapsed, the reaction product was analyzed to find that theacrolein conversion was 38.9% and the selectivity of 2-ethylhexylacrylate was 91.2%. The water concentration in the recovered reactionmixture was 1.3% by weight and the 2-ethylhexyl acrylate-producing rateper catalyst was 10.76 moles/h.Kg catalyst. When the reaction wascontinued for a further 100 hours, it was found that the acroleinconversion was 39.6%, the selectivity of methyl acrylate was 90.2%, theproducing rate was 10.6 moles/h/Kg catalyst, and a constant reactionperformance was maintained.

Example 6

The reaction was conducted in the same manner as in Example 3, exceptthat ethyl alcohol was substituted for the methanol. When 10 hours hadelapsed, the reaction product was analyzed to find that the acroleinconversion was 54.3% and the selectivity of ethyl acrylate was 93.4%.The water concentration in the recovered reaction mixture was 2.1% byweight and the ethyl acrylate-producing rate per catalyst was 15.1moles/h.Kg catalyst. When the reaction was continued for a further 100hours, it was found that the acrolein conversion was 53.9%, theselectivity of ethyl acrylate was 93.6%, the producing rate was 15.0moles/h/Kg catalyst, and a constant reaction performance was maintained.

Example 7

The reaction was conducted in the same manner as in Example 1, exceptthat ethyl alcohol was substituted for the methanol. When 10 hours hadelapsed, the reaction product was analyzed to find that the methacroleinconversion was 46.3% and the selectivity of ethyl methacrylate was92.2%. The water concentration in the recovered reaction mixture was1.21% by weight and the ethyl methacrylate-producing rate per catalystwas 10.3 moles/h.Kg catalyst. When the reaction was continued for afurther 100 hours, the reaction product was analyzed to find that themethacrolein conversion was 45.0%, the selectivity of ethyl methacrylatewas 93.1%. The water concentration in the recovered reaction mixture was1.26% by weight and the ethyl methacrylate-producing rate per catalystwas 10.1 moles/h.Kg catalyst.

Comparative Example 2

In the same apparatus as in Example 3, except that the separationmembrane was not installed, the reaction was conducted under the sameconditions as in Example 3. When 10 hours had elapsed, the reactionproduct was analyzed to find that the acrolein conversion was 35.1% andthe selectivity of methyl acrylate was 88.1%. The water concentration inthe recovered reaction mixture was 3.4% by weight and the methylacrylate-producing rate per catalyst was 9.2 moles/h.Kg catalyst. When100 hours had elapsed, the reaction product was analyzed to find thatthe acrolein conversion was 34.8% and the selectivity of methyl acrylatewas 88.6%. The methyl acrylate-producing rate per catalyst was 9.2moles/h.Kg catalyst.

Comparative Example 3

In the same apparatus as in Example 6, except that the separationmembrane was not installed, the reaction was conducted under the sameconditions as in Example 6. When 10 hours had elapsed, the reactionproduct was analyzed to find that the acrolein conversion was 28.1% andthe selectivity of ethyl acrylate was 87.4%. The water concentration inthe recovered reaction mixture was 2.9% by weight and the ethylacrylate-producing rate per catalyst was 7.3 moles/h.Kg catalyst. When100 hours had elapsed, the reaction product was analyzed to find thatthe acrolein conversion was 27.4% and the selectivity of ethyl acrylatewas 87.8%. The ethyl acrylate-producing rate per catalyst was 7.2moles/h.Kg catalyst.

Comparative Example 4

In the same apparatus as in Example 4, except that the separationmembrane was not installed, the reaction was conducted under the sameconditions as in Example 4. When 10 hours had elapsed, the reactionproduct was analyzed to find that the acrolein conversion was 24.5% andthe selectivity of n-butyl acrylate was 87.6%. The water concentrationin the recovered reaction mixture was 2.3% by weight and the n-butylacrylate-producing rate per catalyst was 6.4 moles/h.Kg catalyst. When100 hours had elapsed, the reaction product was analyzed to find thatthe acrolein conversion was 23.2% and the selectivity of n-butylacrylate was 88.3%. The n-butyl acrylate-producing rate per catalyst was6.1 moles/h.Kg catalyst.

Comparative Example 5

In the same apparatus as in Example 5, except that the separationmembrane was not installed, the reaction was conducted under the sameconditions as in Example 5. When 10 hours had elapsed, the reactionproduct was analyzed to find that the acrolein conversion was 20.2% andthe selectivity of 2-ethylhexyl acrylate was 88.9%. The waterconcentration in the recovered reaction mixture was 2.0% by weight andthe 2-ethylhexyl acrylate-producing rate per catalyst was 5.4 moles/h.Kgcatalyst. When 100 hours had elapsed, the reaction product was analyzedto find that the acrolein conversion was 19.7% and the selectivity of2-ethylhexyl acrylate was 89.5%. The 2-ethylhexyl acrylate-producingrate per catalyst was 5.2 moles/h.Kg catalyst.

Comparative Example 6

In Comparative Example 1, the reactivity after 10 hours was evaluated,and thereafter, 40 g of molecular sieve 4A which had been pulverized to30-100 microns and calcined at 300° C. for 3 hours was added, and after3 hours, the reaction product was analyzed to find that the methacroleinconversion was 55.2% and the selectivity of methyl acrylate was 93.4%.The water concentration in the recovered reaction mixture was 1.4% byweight and the methyl methacrylate-producing rate per catalyst was 12.5moles/h.Kg catalyst. When 100 hours had elapsed, the reaction productwas analyzed to find that the methacrolein conversion was 28.5% and theselectivity of methyl methacrylate was 90.2%. The water concentration inthe reaction mixture was 3.3% by weight and the methylmethacrylate-producing rate per catalyst was 6.2 moles/h.Kg catalyst.

Industrial Applicability

According to the process of this invention, water can be stably andcontinuously removed from the reaction system. As a result, theselectivity of a methacrylic acid ester or an acrylic acid ester and theproducing rate can be enhanced. Also, the process of this invention issuch a simple process that the regeneration of the water-adsorbent isnot required, so that the productivity for a long period of time ishigh, the reactor can be made small, and a small amount of catalyst issufficient. Thus, this invention can improve greatly the economicalefficiency of the production of a methacrylic acid ester or an acrylicacid ester and is very useful in industry.

What is claimed is:
 1. A process for producing a methacrylic acid esteror an acrylic acid ester comprising: reacting methacrolein or acroleinwith an alcohol and molecular oxygen in the presence of a catalystcomprising Pd; removing water with a separation membrane which canselectively permeate water from a mixed liquid of the alcohol and water.2. The process according to claim 1, wherein the separation membrane isan inorganic membrane.
 3. The process according to claim 2, wherein theinorganic membrane has a water-separation factor (α) of at least 1,000and a permeation flux (Q) of at least 0.01.
 4. The process according toclaim 2, wherein the inorganic membrane is an A type zeolite membrane.